The invention relates to a steering system for an amphibious vehicle and to an amphibious vehicle having such a steering system. The invention also relates to a method of modifying a conventional hydraulically power assisted steering rack for use in such a steering system for an amphibious vehicle.
It is already known to provide amphibious vehicles with steering systems capable of operation on land and water. Most simply, these systems are composed of two independent steering arrangements, for example, a steering wheel which is connected to a conventional rack and pinion arrangement for land operation, and a tiller, which is connected to the head of a rudder for water operation.
In more advanced steering systems, a steering wheel controls both land steeling and water steering. For example, in U.S. Pat. No. 5,727,494, an amphibious vehicle is disclosed having a steering wheel connected to a fixed gear box by means of a shaft. The fixed gear box is telescopically connected to a moving gear box, and steering rods connected to the moving gear box enable turning of the front wheels of the vehicle. The steering shaft is also connected to a mechanical cable steering system which controls turning of a rudder. The land and water steering arrangements are both permanently connected to the steering wheel. It is considered that a cable steering system is inflexible in its relation of force to distance, and susceptible to friction and to corrosion in a marine environment.
In accordance with a first aspect of the invention, there is provided a steering system for an amphibious vehicle comprising a steering control operatively connected to a road steering means for steering at least one wheel of the vehicle and a marine steering system, the marine steering system comprising a hydraulic master cylinder in fluid connection with a hydraulic slave cylinder, the slave cylinder being operatively connected to a marine steering means, characterised in that the master cylinder is operatively linked to the road steering means such that it drives the slave cylinder, and hence the marine steering means, in response to movement of the road steering means.
In a preferred embodiment, the master cylinder comprises a double acting piston defining first and second chambers on either side of the piston, the piston being connected to the road steering means for movement therewith, the hydraulic slave cylinder also comprising a double acting piston defining first and second chambers on either side of the piston, the piston of the slave cylinder being connected to the marine steering means for movement therewith, and the first chamber of the master cylinder being in fluid connection with the first chamber of the slave cylinder and the second chamber of the master cylinder being in fluid connection with the second chamber of the slave cylinder.
Preferably, the road steering means is a steering rack which forms part of a rack and pinion assembly. In this arrangement, the master cylinder may be formed as an integral part of the rack and pinion assembly. Part of the casing of the rack and pinion assembly may form a housing of the master cylinder, and the master cylinder piston may be attached to the steering rack for movement with the rack within the master cylinder housing. The master cylinder may be provided in-line with the steering rack. Alternatively, the master cylinder may be arranged substantially parallel to the steering rack, the master cylinder piston being attached to a piston rod which is connected to the rack by means of at least one link.
Preferably, means are provided to selectively activate or de-activate the marine steering system. The means for selectively activating or de-activating the marine steering system may comprise valve means for selectively connecting the chambers of the master cylinder and/or the slave cylinder with a hydraulic fluid reservoir. Alternatively, the means for selectively activating or de-activating the marine steering system may comprise valve means for selectively placing the first and second chambers of the master cylinder in fluid connection with each other.
Where means are provided to selectively activate or de-activate the marine steering system, means may also be provided for synchronising the straight ahead positions of the road steering means and the marine steering means while the marine steering system is activated.
Preferably, tie valve means is operable to activate the marine steering means only when the road steering means is in a straight ahead position or substantially so. A sensor may be provided to detect when the road steering means is in, or substantially in, a straight ahead position, and the steering system may further comprise control means which permits operation of the valve means to activate the marine steering system only when the sensor detects that the road steering means is in, or is substantially in, the straight ahead position. In a preferred arrangement, the sensor detects the position of the master cylinder piston in order to determine when the road steering means is in the straight ahead position.
In an alternative arrangement the master cylinder is connected to a hydraulic fluid reservoir via at least one port located at, or substantially at, a predetermined point in the stroke of the piston which equates with a straight ahead position of the road steering means, the arrangement being such that following actuation of the valve means to actuate the marine steering means, movement of the slave cylinder piston in a given direction will only occur once the piston has passed the central point in its stroke to close a respective chamber from the reservoir. Preferably, the master cylinder is connected to the reservoir via two ports arranged one on either side of the central point of the stroke of the master cylinder piston. The two ports may be spaced by a distance which is slightly larger than the thickness of the master cylinder piston.
Preferably, the steering system further comprises resilient means for biasing the marine steering means towards a straight ahead position. The resilient means may comprise a pair of springs, a first of the springs being located in a first chamber of the slave cylinder and a second of the springs being located in a second chamber of the slave cylinder, the arrangement being such that the first and second springs act on opposite sides of the slave cylinder piston in order to bias the piston towards a predetermined point in its stroke which equates to a straight ahead position of the marine steering means.
Preferably, the road steering means is power assisted. The power assistance may comprise a hydraulic power assistance system. Alternatively, the power assistance may comprise an electric, electro-hydraulic, or magnetic power assistance system. It is particularly advantageous if the road steering system uses hydraulic fluid, as its fluid reservoir can also be used for the marine steering system.
In accordance with a second aspect of the invention, there is provided an amphibious vehicle, characterised in that the vehicle comprises a steering system in accordance with the first aspect of the invention.
In accordance with a third aspect of the invention, there is provided a method of adapting a conventional hydraulically power assisted steering rack for use in a steering system in accordance with the first aspect of the invention, the method comprising:
providing a steering rack having a double acting hydraulic ram defining first and second ram chambers and a control valve;
disconnecting the first and second ram chambers from the control valve;
providing a slave cylinder for actuation of a marine steering means, the slave cylinder having a piston defining first and second chambers on either side of the piston; and
fluidly connecting the first ram chamber of the steering rack with a first chamber of the slave cylinder and fluidly connecting the second ram chamber of the steering rack with a second chamber of the slave cylinder.